Dr Godelieve Kranendonk, a behavioural biologist leading the study at AAP, a rescue centre for animals in the Netherlands, told BBC News that the results had been astonishing.

"Suddenly, [the chimps] woke up. It was as if they were zombies in their enclosures and now they are happy, playing with each other. They are chimps again - that was really nice to see," she told me.

AAP is a rescue centre for Dutch chimps and other mammals that have been used in scientific research.

Many animals emerge from their time in laboratories depressed and traumatised. Having been confined for 15 to 20 years as lab animals, they have lost their ability to play or relate to other chimps. Instead, they spend their time in brooding isolation and sometimes eat their own vomit.

The chimps often repeatedly rock back and forth, pace back and forth and pull their own hair.

Playfulness restored

Staff at the AAP sanctuary care for the animals until they die. They try to rehabilitate them so that they can live out their remaining years happily.

The chimps are fed a good diet of vegetables, have toys and plenty of space in which to play. But Dr Kranendonk found that the abnormal behaviour actually increased. It was as if the animals did not know how to cope with their new found freedom.

Dr Kranendonk decided to consult Martin Bruene, a professor of human psychiatric disorders at the University of Bochum, Germany. He prescribed a course of anti-depressants for five of the chimps.

All the animals had been used in medical experiments and were infected with Hepatitis C. "Willy" showed the least abnormal behaviour. "Tomas" and "Zorro", on the other hand, would spend a third of their waking hours eating their own vomit.

"Iris" had lost so much weight from vomiting when she first came to the sanctuary that the staff thought she would die.

The most troubled though was "Kenny", a small chimp who was constantly anxious that the others would attack him and spent much of his time screaming in terror.

The chimps were given SSRI (Selective Serotonin Reuptake Inhibitors), which is a class of anti-depressant similar to Prozac and is used to treat human patients for depression, anxiety disorder and obsessive compulsive disorder.

After six to eight weeks, the animals behaviour started improving. The abnormal behaviour declined and the chimps began to play together. After seven months, there was a vast difference.

Kenny responded best of all to the treatment. He is now the clown of the group, entertaining the others and initiating play.

Prof Bruene said that the results were "quite amazing".

He said: "I didn't expect this to work this well. These chimps have served as laboratory chimps for many, many years and suffered psychological trauma. I wouldn't expect a human [to recover] that has suffered a similar condition."

After decades of being research animals, the chimps unsurprisingly looked weary and dishevelled, even after their anti-depressant treatment.

Gone was the energy and playfulness that one sees in wild chimps. But they had an engagement that was absent from other chimps in the sanctuary that had not received the medication.

Kenny, for example, came to the front of his cage to show me the carrots he was about to eat. Then, after noticing that I was taking what he may have though to be too great an interest in his dinner, he scuttled off quickly, flashing a suspicious glance as he clutched his carrots tightly to his chest.

And after dinner, Iris and Zorro played together affectionately, even exchanging a kiss.

The big question though is whether the effect lasts when the chimps are taken off the medication. The early indications are promising. The medication has been steadily reduced and there has been no adverse effect on the chimps' behaviour.

Kenny himself decided that he did not want to take the anti-depressants anymore. His clownish behaviour has continued.

"It seems that while on the medication, the chimps learn to be chimps again," said Dr Kranendonk. "And once they have learned that, they don't need the medication any more."

American decision

The result comes at a time when the world's largest user of chimps in medical research is considering retiring more than 300 of its animals.

An expert group of scientists has suggested that the chimps should be sent to a national sanctuary which has been dubbed "Chimp Haven". The 80-hectare (200-acre) site in Louisiana was opened in 2005 as a retirement home for research chimps that were no longer needed.

The proposal from the NIH committee is the latest step in a shift away from using chimps for medical research in the US.

But the experience of Dr Kranendonk and Prof Bruene suggests that simply releasing the chimps into a large open air sanctuary will not be enough. Unless the animals are properly supported, Chimp Haven may well turn out to be Chimp Asylum.

Prof Bruene believes that the NIH should consider giving their animals anti-depressants if they show symptoms of depression before releasing them. He thinks their conditions are treatable and that, as our closest relatives in the animal kingdom, they deserve proper psychiatric care.

"It could be a good model for other facilities to at least try and improve their conditions," he said.